Electronic equipment such as computers, wireless devices, broadband devices, radios, televisions and other similar devices communicate with each another by transmitting signals through air, space and guided media such as wire, cable, microstrip, waveguide, and optical fiber. These transmission signals undergo a variety of processes throughout their communication paths, one of which involves amplifying the signals using power amplifiers.
A radio frequency (RF) power amplifier is a circuit that is capable of receiving an RF input signal and amplifying it to produce an RF output signal that is a magnified version of the input signal. RF power amplifiers are frequently used in communications systems such as wireless telephony, satellite links, optical transceivers, and cable television distribution systems. An RF amplifier typically includes at least one power-amplifying device, such as a power-amplifying transistor, and a bias circuit that sets a quiescent operating point of the transistor. The transistor may be a field effect transistor (FET) device, such as a laterally-diffused metal-oxide-semiconductor (LDMOS) transistor, or a bipolar junctions transistor (BJT) device, such as a heterojunction bipolar transistor (HBT).
In a transistor-based RF amplifier, there are tradeoffs between maximizing efficiency and preserving the fidelity of the RF signal. The efficiency of the amplifier is defined as the output RF signal power divided by the power supplied to the amplifier from a power supply. The fidelity of an amplified signal is often described in terms of a deviation from an ideal linear noiseless process and is characterized using a variety of metrics including harmonic distortion, intermodulation distortion, adjacent channel power ratio (ACPR), cross-modulation, error vector magnitude, and bit error rate, etc., most of which are interrelated. For communication systems involving a modulated carrier signal whose modulation frequency is a small fraction of the carrier frequency, odd-order intermodulation distortion (IMD) processes are the primary sources of deleterious signal distortion in power amplifiers. In a given communication system, the contributing effects of such distortion can be related to other performance metrics such as bit error rate and error vector magnitude. Thus, in the following description, odd-order IMD and its related ACPR characteristic are mainly used as the metrics for the fidelity of amplified signals. See Cripps, “Power Amplifiers for Wireless Communications,” Artech House: Norwood, Mass., 1999, and Pedro and Carvalho, “Intermodulation Distortion in Microwave and Wireless Circuits,” Artech House: Norwood, Mass., 2003.
Given the inherent tradeoff between efficiency and linearity (fidelity), practitioners in the art of power amplifier design have developed a broad portfolio of circuit concepts and methods for exercising this tradeoff. One primary technique for increasing the efficiency of a power amplifier is to control the conduction angle of the transistor device. Various classes of amplifiers have been devised to manage the conduction angle, such as class-A, class-AB, class-B, class-C, class-D, class-F, and class-S amplifiers, listed roughly in the order of reduced conduction angle. See Clark and Hess, “Communication Circuits: Analysis and Design,” Wiley: New York, 1971, and H. L. Krauss et al., “Solid State Radio Engineering,” Wiley: New York, 1980.
The reduced conduction angle allows the amplifier to approach the efficiency of a switching device at the expense of signal distortion and gain. Furthermore, BJT operation at high power (e.g., 2-10 W) and high voltage (e.g., >10V) requires substantial thermal ballasting in either or both of the base and emitter terminals of the power-amplifying transistor. See Anholt, “Electrical and Thermal Characterization of MESFETs, HEMTs, and HBTs,” Artech House: Norwood, Mass., 1995. Such thermal ballasting helps to increase the current-handling capability and thermal stability of the power-amplifying device but at the same time degrade the linearity of the amplifier circuit. See Pedro and Carvalho, supra, and Vuolevei and Rahkonen, “Distortion in RF Power Amplifiers,” Artech House: Norwood, Mass., 2003. The engineering challenge in modern RF power amplifier design is to devise circuits with an optimum configuration of transistor device, tuning, conduction angle and bias control to maximize efficiency while controlling odd-order distortion processes to meet the linearity specifications for a particular communication signal or system.